Bladder cancer is the fourth most common cancer in men and eleventh most common in women. In addition, seventy percent of patients suffer disease recurrence after surgical treatment. Therefore, the development of efficient therapeutics against this pathology is a high priority. The long-term objective of this project is to develop a strategy to target drug- loaded nanocarriers to bladder tumor cells using Fibronectin Attachment Protein (FAP) from the therapy adjuvant Bacillus Calmette-Guerin (BCG). Indeed, we recently showed that microclustering of Fibronectin:Integrin complexes by multivalent FAP-nanocarriers induced nanocarrier uptake by human bladder tumor cells under bladder-like conditions. Here, we will test the hypothesis that multivalency and membrane fusogenic properties of topotecan-loaded FAP-nanocarriers control their uptake and cytotoxicity, respectively. The following specific aims will be pursued using bladder tumor cells in culture and an orthotopic mouse model: 1. Determine the effect of multivalency on FAP-nanocarrier internalization by bladder tumor cells. 2. Determine in bladder tumor cells the cytotoxicity of topotecan-loaded FAP-nanocarriers with different fusogenic characteristics. This project is innovative because utilizes novel and efficient targeting agent, FAP from BCG, for the development of a therapeutic strategy against bladder cancer. Further, we devised a method for promoting the internalization of FAP-nanocarriers by bladder tumor cells. This microclustering-based approach induces FAP uptake through a mechanism with known kinetics, trafficking route and average vesicle-size. In addition, our approach is more advantageous than other more conventional nanocarrier strategies (like the ones using RGD peptides for targeting) because, as previously described, FAP elicits an anti-tumor immunoresponse in immunized individuals. Also, as opposed to RGD peptides, FAP binds to Fibronectin:Integrin complexes rather than targeting low abundance free Integrins or competing off Fibronectin. Further, FAP is not likely to contribute to Integrin-signaling as it binds pre-existing Fibronectin:Integrin complexes. In fact, our strategy leads to lysosomal targeting and degradation of putative Fibronectin:Integrin signaling complexes.